The invention relates to a microlithography projection objective for imaging into an image plane a pattern arranged in an object plane.
The invention further relates to a projection exposure machine having such a projection objective.
The invention relates, furthermore, to a method for producing semiconductor components and other finely structured subassemblies.
Microlithography projection objectives are used in projection exposure machines for producing semiconductor components and other finely structured components, particularly in wafer scanners and wafer steppers. Such projection exposure machines serve the purpose of projecting patterns from photomasks or graticules, which are also generally designated as masks or reticles, onto an object (substrate) coated with a photosensitive layer with very high resolution. In this case, the mask is arranged in the object plane, and the substrate in the image plane of the projection objective.
Known among projection objectives are ones which exhibit a combination of refracting and reflecting optical elements, thus in particular a combination of lenses and mirrors. Such projection objectives are denoted as catadioptric.
An example of a catadioptric projection objective is disclosed in document DE 101 27 227 A1. A further example of a catadioptric projection objective is to be gathered from document WO 2004/019128 A2.
For example, the catadioptric projection objective disclosed in DE 101 27 227 A1 has, starting from the object plane, a first objective part and a second objective part adjacent thereto, and a third objective part adjacent thereto. Beam deflection takes place in the transition from the first objective part to the second objective part by means of a beam deflecting device that is formed there by a first folding mirror. The second objective part has a concave mirror which retroreflects the light again to the beam deflecting device, and the beam deflecting device, which has a further folding mirror in the transition from the second objective part into the third objective part, then directs the light into the third objective part. The two folding mirrors are at an angle of approximately 90° to one another. Moreover, in the case of this known projection objective the optical arrangement is made such that an intermediate image is produced in the third objective part.
Not only is it possible within the meaning of the present invention for a beam deflecting device to be formed by folding mirrors, but the beam deflecting the device can have, for example, a beam splitter cube or other optical elements suitable for beam deflection.
A problem that can arise in the case of projection objectives with beam deflection is that a portion of the light passing through the projection objective leaks at the beam deflecting device directly from the first objective part into the third objective part by omitting the second objective part. Such stray light or false light therefore does not traverse all the optical elements of the projection objective and is thus incapable of correctly imaging into the image plane of the projection objective the pattern arranged in the object plane of the projection objective, since the projection objective is designed such that only, light that traverses all the optical elements in the prescribed sequence can contribute to proper imaging.
DE 101 27 227 A1 proposes arranging a stray light stop in the region of the intermediate image in order to reduce stray or false light. However, this does not effectively alleviate, still less eliminate, the problem of the partial direct light leakage from the first objective part into the third objective part.
As already mentioned above, microlithography projection exposure machines are designed as steppers or as scanners. In the case of steppers, a square or rectangular field is exposed on the stationary wafer. A round field would have the effect of being unable to make use of the complete wafer surface and is therefore not used in facilities for the mass production of semiconductors.
In the case of scanners, the pattern (reticle) and the wafer move, the field exposed on the wafer and the reticle being square or rectangular. Scanners are preferably used in the mass production of semiconductors.
Although rectangular field shapes are preferred for reasons of later process steps, in particular the division of the wafer into individual pieces, for reasons of production engineering projection objectives for use in steppers and scanners are generally constructed from round lens elements.
In the case of a rectangular field exposure, as well, false light occurs that makes no contribution to the imaging of the projected pattern. In addition to the above-described effect in the case of projection objectives having beam deflection, in the case of which false light leaks from one objective part into another objective part, while omitting specific objective parts, false light can also arise in general from reflections at individual lens surfaces and at the surfaces of the pattern (reticle) of the wafer, such reflections failing to vanish despite the use of antireflective layers on the lens surfaces. false light also arises through scattering at the surfaces and in the volume of the lenses.
Irrespective of the particular cause, false light disturbs the lithographic process as soon as it falls onto the wafer since the structures imaged by the exposure process widen owing to the false light background. In other words, false light that is propagating in the projection objective and reaches the image plane of the projection objective disturbs the contrast of the imaging of the pattern into the image plane.
As already described above, a projection objective operates properly in terms of function only when the imaging beam path that can be used for imaging traverses all the optically operative surfaces in a predetermined sequence. False images arise in a projection objective owing to false light that does not traverse all the optically operative surfaces of the projection objective, or that while actually traversing all the optically active surfaces of the projection objective does so in a sequence other than is required for proper imaging. In order for the false light to reach the wafer, an even number of additional reflections must take place. The additional reflections begin with the reflection at a lens surface that is designed as refracting. Hereafter, the light can be retroreflected at other lens surfaces or, in the case of catadioptric projection objectives, at mirrors.
The optical system that belongs to this novel beam path is designated as an “expanded” optical system. Such an “expanded” system images just like the actual projection objective. The false light that contributes to this imaging is certainly weak, since it arises chiefly from a reflection at a refracting surface that mostly also has a layer for reducing reflection.
The “expanded” optical system of the projection objective guides the false light in the direction of the wafer such that a false image is produced in the vicinity of the image plane of the projection objective.
In any event, false light in projection objectives worsens the imaging properties of the projection objective such that there is a need for projection objectives in which false light is suppressed as effectively as possible.
Still another problem in microlithography projection objectives arises in connection with the requirement to provide a uniform aperture over the entire used field. In projection objectives for microlithography, a vignetting, as it is known, for example, in objectives for photography, is undesired. In projection objectives known from prior art it is common to provide a single aperture stop in a pupil plane of the projection objective. However, there are optical designs in which a single aperture stop cannot guarantee the desired uniform aperture over the used field.
The reason for a non-uniform aperture over the field is that there are some rays which start from the object plane with an aperture which is larger than the design aperture of the projection objective and which are then aberrated so strongly that their actual pupil plane is spaced apart from the pupil plane which belongs to rays having apertures which are equal to or smaller than the design aperture. Accordingly, these so-called over-aperture rays are not necessarily mashed out by the system aperture stop, but can reach the image plane if the optical components have a sufficient extension in direction transverse to the propagation of light. These over-aperture rays are typically strongly aberrated and, when reaching the image plane, disturb the uniformity of the image field.
In addition or independent from the problems mentioned before, it can be an object in microlithography projection objectives to mask out light in the beam path of the projection objective such that the image plane remains completely unexposed. A motivation for doing so may be the measuring of the diffraction intensity distribution in order to obtain information for lease heating corrections. Further, during an interruption of the exposure process it is conceivable to supply the projection objective with an illumination which is complementary to the light used for imaging in order to produce a heating of the projection objective which is as uniform and rotationally symmetrical as possible, for example, if this is desired. In this case, too, it must be made sure that no light enters the image plane.